1. Field of the Invention
This invention relates to an improved position transducer and more particularly to method and apparatus for indicating the position of a core member moved relative to a variable differential transformer.
2. Description of the Prior Art
One type of position transducer used in the prior art is a linear voltage differential transformer (LVDT). Typically, such a sensor generates an electrical signal proportional to the displacement of the transformer core with respect to a reference position in response to an external physical motion. The transformer core is generally of high permeability material which couples AC signals induced in the transformer primary into secondary windings of the transformer. The secondary voltage is attenuated by the core position and this induced voltage is either in phase or 180.degree. out of phase with the excitation. In this manner the relative movement of the core controls the output of a modulator/demodulator network.
Normally in position transducers employing an LVDT the demodulators use precision full wave rectifiers and filters as described, for example, in U.S. Pat. No. 4,140,998. Recently the need for an LVDT in a digital system has become apparent. Since all analog to digital converters utilized in current digital systems are relatively fast, i.e., conversion of &lt;30 .mu.S, as compared to typical LVDT excitation frequencies, the lack of adequate demodulator output filtering could lead to system instability due to positive feedback. This positive feedback results from analog to digital conversion of the demodulated and filtered LVDT secondary waveform during the peak and subsequently the valley of the ripple. Thus, the system time constant is dictated by the tolerable ripple and is usually a minimum of twice the period of the excitation frequency.